Method and apparatus for analyzing and/or repairing of an EUV mask defect

ABSTRACT

The invention relates to a method for analyzing a defect of a photolithographic mask for an extreme ultraviolet (EUV) wavelength range (EUV mask) comprising the steps of: (a) generating at least one focus stack relating to the defect using an EUV mask inspection tool, (b) determining a surface configuration of the EUV mask at a position of the defect, (c) providing model structures having the determined surface configuration which have different phase errors and generating the respective focus stacks, and (d) determining a three dimensional error structure of the EUV mask defect by comparing the at least one generated focus stack of the defect and the generated focus stacks of the model structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of PCT/EP2011/060626,filed on Jun. 24, 2011, which claims priority to German Application 102010 025 033.3, filed on Jun. 23, 2010, herein incorporated by referencein its entirety.

1. FIELD OF THE INVENTION

The present invention relates to the field of analyzing and/or repairingof an EUV mask defect.

2. BACKGROUND OF THE INVENTION

As a result of the shrinking sizes of integrated circuits,photolithographic masks have to project smaller and smaller structuresonto a photosensitive layer i.e. a photoresist dispensed on a wafer. Inorder to enable the decrease of the critical dimension (CD) of thestructure elements forming the integrated circuits (ICs), the exposurewavelength of photolithographic masks has been shifted from the nearultraviolet across the mean ultraviolet into the far ultraviolet regionof the electromagnetic spectrum. Presently, a wavelength of 193 nm istypically used for the exposure of the photoresist on wafers. As aconsequence, the manufacturing of photolithographic masks withincreasing resolution is becoming more and more complex.

In the future, photolithographic masks will use even smaller wavelengthsin the extreme ultraviolet (EUV) wavelength range of the electromagneticspectrum. The term EUV mask denotes in the following a photolithographicmask for the EUV wavelength range (preferably 10 nm to 15 nm).

The optical elements for the EUV wavelength range will preferably bereflective optical elements. For the fabrication of an EUV opticalelement a multilayer structure or a multilayer film is deposited on asubstrate having an ultralow thermal expansion (ULE). Fused silica is anexample of a substrate used for EUV optical elements. The multilayersystem typically comprises 80 to 120 alternating layers of molybdenum(Mo) and silicon (Si). A pair of a Mo—Si layer or a Mo—Si bilayer has adepth of approximately 7 nm. At the boundary of the Mo—Si layers aportion of the incident EUV radiation is reflected, so that a Mo—Simultilayer layer system ideally reflects more than 70% of the incidentEUV radiation.

In addition to the multilayer structure, an EUV mask comprises a patternor an absorbing pattern structure on top of the multilayer. For example,the EUV radiation absorbing pattern can be formed of titanium nitride,tantalum nitride, or chromium. The interaction of the EUV radiationabsorbing portions and EUV radiation reflecting portions of the EUV maskgenerates in case of an illumination with EUV radiation the pattern tobe presented in the photoresist dispensed on a semiconductor wafer.

Highest precision is required at the fabrication of EUV opticalelements, in particular for EUV masks. Errors in the order of 1 nm canalready cause errors in the image of the pattern structure on the wafer.Mask errors or defects which are apparent on the pattern of the wafergenerated by the mask are called printing errors. Defects of differenttypes can occur at various positions of the EUV mask leading to variouseffects.

EUV inspection and review systems operating at the illuminationwavelength are already known. The U.S. Pat. Nos. 6,954,266, or 5,808,312describe EUV inspection and review systems or tools operating incombination with a mask repairing system. EUV review systems are alsodenoted as EUV mask inspection microscopes (EUVM).

Further investigation methods for EUV masks are also known. The US2009/0 286 166 discloses the use of an atomic force microscope (AFM) forthe localization of concave defects on EUV masks. The U.S. Pat. No.6,844,272 describes the determination of the height of the surface ofEUV masks by the use of an interferometer.

Defects of the absorbing pattern structure may occur if absorbingmaterial is missing at positions which should be opaque, or whenabsorbing material is existent at positions which should be dear.Further, dirt particles may be attached to the surfaces of EUV opticalelements. This type of error results predominantly in amplitude errors.It can be recognized by a surface analysis of the EUV optical element;for example by using a scanning electron microscope (SEM). Using a knownmask repairing system, as for example the MeRiT® system of Carl ZeissSMS excessive material can be removed. An electron beam in combinationwith a suitable etching gas can be used for this task. Missing absorbermaterial can for example be added by locally depositing chromium withthe aid of an electron beam together with a respective precursor gas.

EUV optical elements can be washed or polished in order to dean theirsurfaces from disturbing particles or substances.

On the other hand, so-called buried defects can occur in EUV opticalelements, i.e. EUV mirrors and/or EUV masks. In the following, the term“buried defect” means a defect or an error which is located on thesubstrate and/or within the multilayer structure of the EUV opticalelement. Buried defects lead to both, amplitude and phase errors, i.e.buried defects comprise amplitude and phase error portions. Burieddefects are also called topological errors.

The U.S. Pat. No. 6,016,357 describes a method for correcting errors ofthe absorbing pattern structure by the measurement of focus stacks inphase shift masks (PSM) illuminated with deep ultraviolet (DUV)radiation. Further, this document describes a repairing method forremoving excessive absorbing materials and for depositing missingabsorbing material. This repairing method is denoted as compensationalrepair.

The U.S. Pat. No. 6,235,434 describes the repair of amplitude and phaseerrors of EUV masks. Independent of the type of error, the repair isdone by compensation, i.e. correcting of the absorbing pattern byremoving excessive material from the absorbing pattern structure ordepositing absorbing material to the absorbing pattern structure,respectively. Further, the US 2005/0 157 384 also discloses the removalof material, whereas the U.S. Pat. No. 6,849,859 describes theadjustment of a thickness of a layer by depositing an additionally layerand by adjusting the thickness of the additional layer.

When excessive absorbing material is removed by an ion beam, ions areimplanted in a buffer layer arranged between the multilayer film and theabsorbing pattern structure. The implanted ions may vary thereflectivity of the corrected EUV mask portion. The JP 2008 085 223 Adiscloses a method to correct the reflectivity change induced by theimplanted ions by respectively correcting the absorbing patternstructure.

The article “Study of critical dimensions of printable phase defectsusing an extreme ultraviolet microscope” by Y. Kamaji et al., Jpn. J. ofAppl. Phys. 48 (2009), pp. 06FA07-1-06FA07-4 explains why pits are moreoften defects in multilayer films of EUV masks than bumps. Further, thearticle reports on the fabrication of programmed phase defects and theiranalysis in order to determine the resolution limit of an EUV microscope(EUVM).

The thesis of C. H. Clifford: “Simulation and compensation methods forEUV lithography masks with buried defects”, Electrical Engineering andComputer Sciences, University of California at Berkeley, Techn. ReportNo. UCB/EECS-2010-62 describes simulation methods which allow generatingsimulation configurations based on aerial images of defects. Thisdocument also reports on two methods for defect compensation byadjusting the absorber pattern of EUV masks.

The article “Natural EUV mask blank defects: evidence, timely detection,analysis and outlook” by D. van den Heuvel et al., SPIE/BACUS Conf.Proc. 2010, describes a method to combine aerial images, marks and AFMmeasurement data in order to localise and to measure an EUV defect whichcannot be recognized in a scanning electron microscope (SEM). Moreover,this paper describes that both, pits as shallow as 3 nm and bumps just 3nm high at the surface can results in critical printing defects buriedin the multilayer.

The above mentioned documents do often not clearly distinguish betweenamplitude errors and phase errors of buried defects, i.e. of defects onsubstrates and/or multilayer films of EUV masks. The repair methodsdenoted as “compensational repair” addresses the amplitude errorportions of buried defects, but ignores their phase error portions. FIG.1 schematically illustrates the compensational repair of a multilayerdefect by removing a portion of the absorbing pattern elements adjacentto the defect in order to compensate for the reduced reflectivity of theburied multilayer defect.

The compensational repair has the drawback that it results in adiminution of the process window at the wafer illumination, since an EUVmask compensated with this approach has a focus dependency whichdeviates significantly from the ideal focus characteristics. Moreover,the compensational repair method cannot be applied to correct defects inEUV mirrors not having an absorbing pattern structure.

It is therefore one object of the present invention to provide a methodand an apparatus for analysing and/or repairing of a defect of an EUVmask, which at least partially overcome the above mentioned drawbacks ofthe prior art.

3. SUMMARY OF THE INVENTION

According to a first aspect of the invention, a method according toclaim 1 is provided. In an embodiment, a method for analyzing a defectof a photolithographic mask for an extreme ultraviolet (EUV) wavelengthrange (EUV mask) comprises (a) generating at least one focus stackrelating to the defect using an EUV mask inspection tool, (b)determining a surface configuration of the EUV mask at the position ofthe defect, (c) providing model structures having the determined surfaceconfiguration which have different phase errors and generating therespective focus stacks, and (d) determining a three dimensional errorstructure of the EUV mask defect by comparing the at least one generatedfocus stack of the defect and the generated focus stacks of the modelstructures.

The described method exploits that amplitude errors and phase errorsbecome manifest in different focus characteristics. Therefore, themeasured focus stack enables discriminating between amplitude errors andphase errors. Amplitude errors basically correspond to surface defectsand phase errors basically corresponds to buried defects. Amplitudeerrors are corrected using a method known in the state of the art.

The surface configuration of an identified buried defect whichdemonstrates in a phase error is analyzed. From the measured surfaceconfiguration model structures are calculated from which a threedimensional (3D) error structure of the defect is determined.

Thus, the described methods define a procedure to reliably detect anddiscriminate defects of an EUV mask and to determine an individual 3Derror structure of identified buried defects. The 3D error structure ofthe buried defect can be used to develop a repairing or a compensationpolicy individually designed for the analyzed buried defect.

An EUV mask is preferably a reflective optical element. However, it isalso possible to apply the method defined in the first paragraph of thissection to transmissive optical elements. Furthermore, the describedmethod is not restricted for analyzing defects of EUV masks. In fact, itcan also be applied to analyze defects of photolithographic masks ormore general of optical elements designed for a longer wavelength rangeas well as for a shorter wavelength range.

In a further aspect determining the surface configuration comprisesscanning the defect with an atomic force microscope, and/or with ascanning tunnelling microscope, and/or with a stylus profilometer,and/or with an interferometer.

After a phase error has been identified and localised by analysing afocus stack measured with a mask inspection tool, the surface of the EUVmask above the buried defect causing the phase error is scanned indetail in order to determine the surface configuration. The surfaceconfiguration corresponds to the surface topology at the defectiveposition of the EUV optical element. The exemplary combination of bothmeasurement data allows the determination of a 3D error structure forthe analyzed defect.

Another aspect comprises the step of applying different repairingmethods to the three dimensional error structure and simulatingassociated focus stacks in order to determine an optimal repairingmethod.

After having determined a 3D error structure, the effect of variousrepairing methods on the resulting focus stack can be evaluated bysimulation prior to the application of the respective repairing method.A repairing method is identified by simulation which is optimallyadapted to the analyzed defect prior to the correction of the defect. Inparticular, the described repairing method is not restricted to amodification of the pattern structure arranged on the multilayer film ofan EUV mask. Rather, the defined repairing method explicitly includes amodification of the substrate and/or of the multilayer film of the EUVmask in order to correct analyzed defects not only of EUV mask but alsoof EUV mirrors.

Therefore, the defined method corrects buried defects instead of justcompensating their amplitude error portion by a compensational repair.

In a further aspect determining the optimal repairing method comprisesselecting one of the different repairing methods as the optimalrepairing method which generates a focus stack which maximizes a processwindow for the EUV optical element at the illumination of a wafer.

The process window is maximized for the repairing method whose aerialimages of the focus stack fulfill a predetermined critical dimension(CD) variation for the largest defocus range of the different repairingmethods. The application of this optimization criterion maximizes theprocess window at the illumination of the corrected EUV optical elementor which maximizes the depth of focus (DOF), which is of utmost interestfor the application of the EUV optical element in a lithography system.

Another aspect comprises applying the optimal repairing method to thedefective position.

The specific treatment of the defective position minimizes the influenceof the defect on the image of the EUV pattern in the photoresistarranged on a wafer, so that the EUV optical element can again beutilized in the production process after the respective error handling.

In still a further aspect, the model structures comprise an absorbingpattern structure on a surface of the EUV mask.

The existence of an absorbing pattern structure facilitates thedetermination of model structures for a defective position, as theabsorbing pattern structure can be used for the identification of thedefect position on the surface of the EUV mask. Further, a modificationof the pattern elements can also be part of the error correctionprocess. On the other hand, it is not mandatory that the modelstructures of a buried defect comprise a pattern structure. Rather, thedescribed method can also be applied to EUV mirrors which do not have anabsorbing pattern.

Another aspect comprises providing the absorbing pattern structure fromEUV mask design data and/or from of a recording of at least one image.

When the pattern data is available, it is used as input for thesimulation process of the effects of the various repairing methods. Onthe other hand, if this information is not available, it canalternatively be obtained from the recording of an image of the patternstructure. A scanning electron microscope (SEM) can be used forrecording an image of the pattern. It is also possible to use acombination of both methods in order to determine the pattern data of anEUV mask.

A further aspect comprises using a repairing method correcting the threedimensional error structure so that a resulting multilayer structure isat least approximately corrected to an ideal multilayer structure.

In contrast to a compensational repair which just modifies patternelements when trying to compensate a buried defect, i.e. a multilayerand/or a substrate defect, the described method modifies the substrateand/or the multilayer in order to correct a buried error. Thus, thedefined method allows correcting analyzed buried defects to a muchhigher amount than by a compensational repair, which just addresses theamplitude error portion of a buried defect. Consequently, the describedmethod maximizes the process window of an EUV optical element at its usein a lithography system.

Another aspect comprises applying the repairing method directly onto thedefective position of the EUV mask.

As already mentioned above, the described method analyzes the 3D errorstructure of a buried defect and corrects the defect by acting on thesubstrate and/or on the multilayer film instead of only partiallycompensating the defect by locally modifying pattern elements close tothe buried defect.

According to another aspect, the repairing method comprises at leastpartially removing the multilayer structure, in particular drilling atleast one hole into the multilayer structure.

By removing the portion of the multilayer structure which comprises theburied defect, the defect is also removed. Then a defect-free multilayerfilm can be newly deposited. Although, this procedure requires someefforts, it does not compensate the defect; rather it is removed by therepairing process.

In still another aspect, the repairing method comprises locallycompacting and/or expanding the multilayer structure and/or of asubstrate of the EUV mask or generally of an EUV optical element bylocally focusing femtosecond laser pulses into the EUV mask.

The defined repairing method is an example of a repairing which directlycorrects a buried defect by acting upon the substrate and/or themulti-layer film of the EUV optical element. In this context, avariation of the substrate is preferred, because the impact of therepair of the defect on the multilayer can be minimized.

In another aspect, the femtosecond laser pulses are incident through thesubstrate of the EUV mask.

When the femtosecond laser pulses are directed through the substrate tothe defective position, the multilayer or at least the most importantupper Mo—Si layers of the multilayer film are not or at least notsignificantly influenced by the correction of the buried defect.

According to a further aspect, an inspection microscope forphotolithographic masks in the extreme ultraviolet wavelength rangeperforms a method of any of the aspects described above.

According to a further aspect of the invention, a method according topatent claim 11 is provided. In an embodiment, a method for repairing aburied defect in an extreme ultraviolet (EUV) optical element comprisesdirecting an ion beam onto the buried defect so that an ion dose isimplanted in the EUV optical element suitable to locally change a volumeof the EUV optical element.

The EUV optical element comprises an EUV mirror having a substrate and amultilayer structure and/or an EUV mask having a substrate, a multilayerstructure and an absorbing pattern structure.

The inventive method corrects buried defects of EUV optical elements bydirectly modifying the substrate and/or the multilayer instead of justcompensating the amplitude error portions of these defects by applying acompensational repair. Thus, the inventive method can not only beapplied to EUV masks, but it can also be used to correct buried defectsof EUV mirrors.

EUV optical elements will probably be reflective optical elements.However, the inventive method is not restricted to reflective opticalelements, but can also be applied to transmissive optical elements.Moreover, the defined method is also not limited for the correction ofburied errors of EUV optical elements. Rather, it can be applied tocompensate buried defects of optical elements designed for longer aswell as for shorter wavelengths.

In still a further aspect, the ion beam comprises inert gas ions,preferably noble gas ions, and most preferably helium ions.

Inert gas ions have the advantage that they do not react with thematerial into which they are implanted. This means, the mechanicaland/or the optical properties of the substrate and/or the multilayerfilm are not significantly modified as a consequence of implanting ions.Further, the noble gas characteristics of the inert gas ions alsoprevents that the EUV photons of the lithography system induce areaction of the implanted inert gas with the surrounding material duringthe operation of the EUV optical element which could change its opticalproperties in the course of time.

A further aspect comprises adapting an ion beam energy to a depth of theburied defect below a surface of the EUV optical element. In yet anotheraspect, the ion beam energy comprises a range of 1 keV to 200 keV,preferably 5 keV to 100 keV, and most preferably of 10 keV to 50 keV.

Ions are implanted in the EUV optical element with a specific spatialdistribution. The spatial distribution as well as the maximum of thedistribution depend on the energy with which the ions hit on the EUVoptical element. The distribution of the implanted ions in beamdirection is in the following called depth distribution. The energy withwhich the ions impinge of the EUV optical element can be selected sothat the maximum of the depth distribution of the implanted ions fits tothe depth of the buried defect.

In still another aspect, the EUV optical element comprises a multilayerstructure and the energy of the ion beam is selected such that the ionsare essentially implanted below layers of the multilayer structure ofthe EUV optical element which determine its reflective properties.

The term “essentially” means here as well as at further positions withinthis specification that the maximum and/or the width of the depthdistribution are adjusted to the respective Mo—Si layers so that theselayers comprise the majority of the implanted ions.

As already mentioned in the second section, ions implanted in themulti-layer structure may locally change the reflectivity of EUV photonsand may such form an amplitude error. It is well known that a multilayerfilm shows an asymptotic reflectance characteristic. This means that theMo—Si layers close to the surface of the EUV element account for themajor portion of the reflected EUV radiation, whereas layers close tothe substrate only insignificantly contribute to the reflectivity of theEUV optical element. Consequently, ions implanted in the lower Mo—Silayers (the Mo—Si layers dose to the substrate) can efficiently correcta buried defect without essentially influencing the reflectiveproperties of the EUV optical element.

In a further aspect, the EUV optical element comprises at least onecapture layer arranged between the substrate and the multilayerstructure, and wherein ion beam energy is adjusted so that the ions areessentially implanted in the at least one capture layer.

By implanting the majority of the ions in the capture layer thereflective properties of the multilayer structure is not or at least notsignificantly changed. A defect located on the substrate surface in theMo—Si layers close to the substrate may propagate through at multilayerand thus affecting its reflective characteristic. Therefore, byimplanting the majority of the ions in a capture layer, a buried defectcan efficiently be removed, so that the defect correction process doesnot significantly influence the reflective properties of the multilayerstructure.

Still a further aspect comprises arranging a local protection layer on asurface of the EUV optical element through which the ion beam isdirected when repairing the defect prior to directing the ion beam ontothe defect and removing the local protection layer at the end of therepairing.

Ions impinging on an EUV optical element have a sputter effect on thesurface of the EUV optical element, i.e. they have an energy to removeatoms from the surface of the EUV optical element. Hence, the impingingions may damage the surface of the EUV optical element. This effect canbe avoided by locally arranging of a protection layer on the multi-layerprior to the ion bombardment and by removing the protection layer at theend of the defect repairing process.

In accordance with a further aspect, the ion beam is not perpendicularlydirected through a surface of the EUV optical element onto the burieddefect.

As already mentioned, beside the sputter effect, ions may induce damagesin the multilayer along their paths from the multilayer surface to theirimplanted position. By using an inclined incidence for the ion beam, aportion of the EUV optical element can be selected for the paths of theions which has less impact on the imaging of the mask pattern on thewafer. In particular, an incidence angle can be chosen so that the ionbeam incidents on an absorbing element. By this approach the multi-layerstructure above a buried defect is not damaged during the repairing ofthe defect.

In yet another aspect, the buried defect is a concave defect, inparticular a pit and/or a scratch and the local volume change comprisesa local volume increase, in particular a local height increase.According to another aspect, the buried defect is a convex defect, inparticular a bump and the local volume change comprises a local volumedecrease, in particular a local height decrease.

Moreover, still a further aspect comprises the step of analyzing theburied defect with a method of any of the aspects described above.

Finally, according to another aspect, a focused ion beam apparatusperforms a method of any of the aspects described above.

4. DESCRIPTION OF THE DRAWINGS

In order to better understand the present invention and to appreciateits practical applications, the following figures are provided andreferenced hereafter. It should be noted that the figures are given asexamples only and in no way limit the scope of the invention.

FIG. 1 schematically represents a compensational repair of a burieddefect of an EUV mask according to the prior art;

FIG. 2 schematically shows a sectional view of an EUV photolithographicmask;

FIG. 3 schematically shows the EUV mask of FIG. 2 having variousdefects;

FIG. 4 schematically depicts a sectional view of the propagation of aburied defect on the substrate through various Mo—Si layers of themultilayer structure;

FIG. 5 schematically depicts a simulation of a critical dimension (CD)variation as a function of an offset (distance between the centerposition of the absorber element and the center of a pit defect) infocus and for defocus positions of ±75 nm;

FIG. 6 schematically shows a sectional view of a cut-out of an EUV maskwith a defect and an offset of the defect with respect to the center ofa pattern element indicating the configuration simulated in FIG. 5;

FIG. 7 schematically represents a height variation of an EUV masksurface as a function of an implanted helium ion dose;

FIG. 8 schematically represents a depth of penetration as a function ofthe helium beam energy in an EUV mask, wherein helium ions hit on themultilayer of the EUV mask;

FIG. 9 schematically represents a depth of penetration as a function ofthe helium beam energy in an EUV mask, wherein helium ions hit on anabsorber pattern element of the EUV mask;

FIG. 10 schematically illustrates the propagation of a buried defectthrough the multilayer structure of an EUV mask (left) and the action ofa volume increase by implanting ions in its multilayer structure;

FIG. 11 schematically shows a EUV optical element having a capture layerarranged between the substrate and the multilayer of an EUV mask (left)and schematically illustrates a depth profile of the implanted ions(right);

FIG. 12 schematically represents a deposition of protection layer on themultilayer over a pit defect of the substrate (left), schematicallyshows the protection layer after the repairing of the pit defect(center), and schematically depicts the EUV mask after finalization ofthe repairing process (right); and

FIG. 13 schematically illustrates the repairing of a pit defect on thesubstrate of an EUV mask by obliquely directing ions through anabsorbing pattern element to a defect (left), and schematically showsthe EUV mask at the end of the repairing process having a damaged volumeby the interaction of ions with the atoms of the multilayer.

5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, the present invention will be more fully describedwith reference to the accompanying figures, in which exemplaryembodiments of the invention are illustrated. However, the presentinvention may be embodied in different forms and should not be construedas limited to the examples set forth herein. Rather, these examples areprovided so that the disclosure will be thorough and will convey thescope of the invention to persons skilled in the art.

FIG. 1 shows a top view of a cut-out of an EUV mask 100. The leftdiagram no comprises a multilayer structure 120 which has a multilayerdefect 130 buried in the multilayer structure 120, which is called aburied defect. On the multilayer 120 two pattern elements 140 arearranged having a form of absorbing stripes. The right diagram 150illustrates the absorber structure 160 of the left diagram no of the EUVmask 100 after the multilayer defect 130 of has been partiallycompensated by removing a portion of the absorbing material of thepattern elements 160 around the buried defect 130 in order to compensatethe reduced reflectance of the area covered by the buried defect 130. Asalready mentioned in the third section, this approach of buried defectrepairing is called “compensational repair”.

FIG. 2 shows a schematic cross-sectional view of a photolithographicmask 200 for an exposure wavelength of 13.5 nm. Different from presentlyapplied photolithographic masks, the EUV mask 200 is a reflectiveoptical element based on a multilayer structure 205. The multilayerstructure 205 acts as a mirror which selectively reflects incident EUVphotons. The multilayer structure 205 of the EUV mask 200 is depositedon a front substrate surface 215 of a suitable substrate 210, such as afused silica substrate. Other transparent dielectrics, glass materialsor semiconducting materials may also be applied as substrates forphotolithographic masks as for example ZERODUR®, ULE® or CLEARCERAM®. Itis preferred that the material of the substrate 210 has a very lowthermal expansion coefficient.

The multilayer film or multilayer structure 205 comprises 40 to 60 pairsof alternating molybdenum (Mo) 220 and silicon (Si) layers 225 (referredto in the following as Mo—Si layers). The thickness of each Mo layer 220is 4.15 nm and that of the Si layer 225 amounts to 2.80 nm. In order toprotect the multilayer structure 205, a capping layer 230 of siliconwith a native oxide of 7 nm depth is arranged on top of the multilayerstructure 205. Other materials can also be used for forming a cappinglayer 230 as for example ruthenium.

In the multilayer 205, the Mo layers 220 act as scattering layers,whereas the silicon layers 225 function as separation layers. For thescattering layers instead of Mo other elements with a high Z number maybe utilized, such as cobalt (Co), nickel (Ni), tungsten (W), rhenium(Re) and iridium (Ir).

As already mentioned, the multilayer structure 205 on the substrate 210of the EUV mask 200 acts as a mirror for EUV electromagnetic radiation.In order to become an EUV mask 200, a buffer structure 235 and anabsorbing pattern structure 240 are additionally deposited on thecapping layer 230. The buffer layer 235 may be deposited to protect themultilayer structure 205 during processing, for example during etchingand/or repairing of the absorbing pattern structure 240. Possible bufferstructure materials are for example of fused silica (SiO₂),silicon-oxygen-nitride (SiON), ruthenium (Ru), chromium (Cr), and/orchromium nitride (CrN). The absorbing structure 240 comprises a materialhaving a large absorption constant for photons in the EUV wavelengthrange. Examples of these materials are chromium (Cr), titanium nitride(TiN) and/or tantalum nitride (TaN).

An anti-reflective (AR) layer 245 can additionally be arranged on theabsorbing pattern structure 240 in order to secure that no photons arereflected by the surface of the absorber pattern 240. A material for anAR layer is for example tantalum oxynitride (TaON). A thickness of about50 nm is sufficient to absorb basically all EUV photons 250 incident onthe absorbing structure 240. In contrast, the majority of the photons250 incident on the capping layer 230 is reflected as photons 255. Inthis context as well as on further positions of this description theterm “basically” means a numeric value of a quantity within itsmeasurement limit.

The substrate 210 of the EUV mask 200 has typical lateral dimensions of152 mm×152 mm and a thickness or height of essentially 6.35 mm. The rearsurface 270 of the substrate 210 or the rear substrate surface 270 has athin metallic coating 275. Typically this coating 275 compriseschromium. The metallic coating 275 is used to fix the EUV mask 200 atthe EUV scanner by the application of electrostatic forces.

FIG. 3 represents the EUV mask 300 of FIG. 2 having various defects. Inorder to keep the following consideration simple, the capping layer 230,the buffer layer 235 as well as the AR layer 245 is omitted in FIG. 3.On the multilayer structure 305 an absorbing pattern 340 is deposited.The pattern element 350 of the absorbing pattern 340 has a portion whichpartially misses the absorbing material. Furthermore, a dirt particle360 is attached on the surface 370 of the multilayer 305. The dirtparticle 360 can be removed by a cleaning process, i.e. by washingand/or polishing the EUV mask 300. Both types of defect, absorberdefects 350 and particle defects 360 on the surface 370 of the EUV mask300 are in the following called surface defects 380.

The surface defects 380 can be detected by a surface analysis of the EUVmask 300. An electron beam of a scanning electron microscope (SEM) canfor example be applied for the surface analysis of the EUV mask 300.Furthermore, the surface defects 380 are accessible to a repair by amodification of the absorbing pattern 340. As already mentioned in thesecond section, this can be performed by using a mask inspection andrepairing system, as for example the MeRiT® system of Carl Zeiss SMS.Such a tool allows adding absorber material to the defective patternelement 350 by using an electron beam in combination with a suitableprecursor gas or a combination of precursor gases. Examples of precursorgases are metal carbonyls, in particular dicobalt octacarbonyl(Co₂(CO)₈).

When the dirt particle 360 can not be removed by a cleaning process, theabsorbing pattern 340 can be modified in order to compensate for thereduced reflectance in the area of multilayer 305 of the EUV mask 300which comprises the dirt particle 360. This can be done by removing aportion the absorbing pattern 340 around the particle defect 360 as itis schematically indicated in FIG. 1 for the compensational repair of aburied defect 130. An electron beam together with an etching gas or acombination of etching gases can be applied in order to selectivelyremove a portion of the absorbing pattern 340. For example, xenondifluroride (XeF₂) can be used as an etching gas.

Further, the EUV mask 300 of FIG. 3 also shows a defect 320 in thesubstrate 310. In the example of FIG. 3, the substrate defect 320 is apit or a scratch on the surface of the substrate 310. The substratedefect 320 propagates as defect 330 in the multilayer structure 305 ofEUV mask 300. During its propagation through the multilayer structure305, the multilayer defect 330 increases its lateral dimension, i.e. thedistortion of the Mo—Si layers towards the multilayer surface extends ona larger area. At the same time, the variation of the height of theindividual Mo—Si layers reduces during the propagation of the multilayerdefect 330 through the multilayer structure 305. As indicated in FIG. 3,the substrate defect 320 of the substrate 310 is reflected in a shallowdepression of the surface 370 of the multilayer 305 above the burieddefect 320. The height variation of the depression at the surface 370 ofthe multilayer 305 may be as small as a few nanometers.

As discussed above, FIG. 3 represents a buried defect 320 caused by apit or a scratch on the substrate 310. It is also possible that there isfor example a dust particle on the surface of the substrate 310 at thebeginning of the deposition of the multilayer 305. Furthermore, thesubstrate 310 may have a small local raise of its surface 370. FIG. 4schematically depicts a cross section of a cut-out of an EUV mask 400wherein a substrate 410 has a bump defect 420. Similar to the pit defect320 of FIG. 3, the bump defect 420 propagates as multilayer defect 430through the multilayer 405.

In addition to the substrate defects 320, 420, buried defects can alsobe localized in the multilayer 305, 405. For example, individual Moand/or Si layers may have defective positions at which the width of oneor several layers may deviate from its target value (not indicated inFIGS. 3 or 4). Moreover, it is conceivable that a dust particle on aspecific Mo and/or Si layer may disturb the periodicity of successivelydeposited Mo—Si layers of the multilayer structure 305, 405. Similar tothe substrate defects 320, 420, these local distortions of theperiodicity of the multilayer structure 305, 405 can propagate towardsthe surface 370, 470 of the multilayer 305, 405.

In the following, defects of a substrate 310, 410 and defects within amultilayer structure 305, 405 are summarized as buried defects 480. Incontrast to surface defects 380 which lead to amplitude errors on awafer, buried defects 480 primarily result in phase errors at waferillumination.

Although the substrate defects 320, 420 as well as the defects withinthe multilayer 305, 405 often become manifest only in a small localdepression or a small local increase of the multilayer surface 370,these defects may cause printable errors on a wafer at its illumination.

The detection of the substrate defects 320, 420 or generally of burieddefects 480 is involved. An electron beam of a SEM can neither detectthe substrate defects 320, 420 nor the shallow depression or increase atthe surface 370, 470 of the multilayer 305, 405 induced by the defects320, 420. For example, an atomic force microscope (AFM) can detect theshallow variation of the surface 370, 470 of the multilayer system 305,405. However, the substrate defects 320, 420 or generally the burieddefect 480 causing a small local increase or depression have alreadybeen identified and localized by another metrology tool. Timeconstraints prevent to scan the overall EUV mask 300, 400 which an AFM.

Recording not only a single aerial image of the defective position of anEUV mask 300, 400 with a mask inspection tool, but recording of a set ofaerial images through focus can reveal the buried defects 320, 420. Forthis purpose, an aerial image of the defective position is recorded inthe best focus plane. Additionally, aerial images are recorded with adistance above and below to the focal plane, as for example ±15 nm, ±30nm, ±45 nm, ±60 nm, and ±75 nm. In this example, the focus stackcomprises 11 images. The number of images as well as the defocusdistance can be adjusted to the analyzed defect.

FIG. 5 shows a diagram 500 of a simulation of the critical dimension(CD) variation as a function of the offset which is the distance betweenthe center 650 of an absorbing pattern element 640 and the location 660of the center of the defect 630. FIG. 6 illustrates the configurationsimulated in FIG. 5. FIG. 6 depicts a multilayer structure 605 having aburied defect 620 similar to the bump defect 420 of FIG. 4. The bumpdefect 620 propagates as defect 630 towards the multilayer surface 680.The multilayer surface 670 has an absorbing pattern element 640. Thecenter 650 of the absorbing pattern element 640 is the reference point650 of the offset of or the distance to the center 660 of the defect630. A shift of the buried defect 620 in the left direction in FIG. 6results in FIG. 5 in a positive offset and vice versa.

In FIG. 5 the CD amounts to 22 nm as indicated by the vertical dashedline 510 at this CD numerical value. An upper dashed line 520 and thelower dashed line 530 represent boundary lines of a CD variation of±10%. The curve 540 represents a simulation of the CD variation in focusas a function of the distance of the defect 620 from the center 650 ofthe pattern element 640. For an offset in the range between 60 nm and 80nm the simulated CD variation is larger than 2.2 nm, i.e. the CDvariation is larger than 10%. The curve 550 shows the simulated CDvariation as a function of the offset for a defocus of +75 nm. For thisdefocus, the CD variation is within the ±10% CD variation bandwidthacross the overall simulated offset. On the other hand, the curve 560simulated with a defocus of −75 nm is for the larger portion of thesimulated offset range beyond the tolerable CD variation of ±10%.

The simulated curves 540, 550, 560 indicate a strong dependence of theCD variation of the focus position. Such a behaviour is expected for aphase error as is indicated by the buried defect 620 in FIG. 6. FIG. 5demonstrates that the measurement of aerial images of a focus stack orthrough focus can be used to discriminate between amplitude errors ofsurface defects 380 and phase errors of buried defects 480 of EUV masks300, 400, 600.

The curves 540, 550 and 560 of FIG. 5 can be used for a compensationalrepair. An improvement of the in focus curve 540 can be obtained bymodifying the pattern element 640 as schematically indicated in FIG. 1,so that the in focus curve 540 fulfils the ±10% variation criterionacross the overall offset range. However, the CD variation of thedefocus curves 550 and/or 560 may still be beyond the ±10% variationcriterion. Thus, such a repairing diminishes the process window at theillumination of a wafer with the EUV mask 600. This situation isunsatisfactory; therefore a defect recognition method is necessary whichanalyzes a buried defect in detail. Further, a defect repairing methodis required which repairs buried defects in such a way that the processwindow is as large as possible at the end of the repairing process.

The surface configuration of the EUV mask 300, 400, 600 is measuredaround the identified buried defect 320, 420, 620 for example with anatomic force microscope (AFM). Further tools which can be applied toscan the defective position are for example a scanning tunnellingmicroscope, a stylus profilometer, and/or an interferometer. Themeasured surface configuration allows the determination of the deviationof the measured surface configuration from an ideal surfaceconfiguration for the EUV masks 300, 400, 600.

In the next step, model structures are generated for various burieddefects of an EUV mask. For the generation of the model structures, thedata of the absorber pattern of the EUV mask at the defective positionis required. This data is obtained from the design data of the mask, orit is obtained from the image recorded with an SEM.

Although the defect analysis is in the following explained for an EUVmask, it is appreciated that the described method can also be appliedfor the defect analysis of EUV mirrors. Moreover, the defined method isnot restricted to the EUV wavelength range, but can also be utilized forthe defect analysis of transmissive optical elements.

For each of the generated model structures aerial images for each focusposition of a focus stack are simulated. In this simulation process, aplurality of the above discussed defects is considered for each surfaceconfiguration.

Then the measured surface configuration and the measured aerial imagesof the focus stack at the defective position are compared with thesimulated aerial images of the focus stacks of the generated modelstructures. The model structure whose aerial images provide the bestagreement with the measured aerial images across the focus stack istaken as the three dimensional (3D) error structure of the identifieddefect.

Finally, based on the determined 3D error structure, a repairing orcompensation policy is developed which minimizes the impact of the 3Derror structure of the EUV mask at the wafer illumination by a specifictreatment of the defective position. The objective of the treatment isto again utilize the EUV mask in a production process for the waferillumination at the end or the repairing process.

Two alternative approaches for the determination of a 3D error structureare conceivable: (a) if the measured aerial images of the focus stackindicate that the identified error is an amplitude error, the surfacedefect 380 causing the amplitude error can directly be corrected byvarying the pattern structure 340, 640. This alternative does notanalyze the surface configuration of the EUV mask around the detectedsurface defect 380 by scanning with an AFM.

(b) Irrespective of the identified error type, the surface configurationof the detected defect in scanned for example with an AFM in order todetermine a surface configuration for a subsequent determination of a 3Derror structure for each identified defect.

For the correction of the determined 3D error structure variousrepairing methods can be applied. As already mentioned above, a portionof an absorbing pattern structure can be removed by using an electronbeam and/or an ion beam and as appropriate in combination with anetching gas or a combination of etching gases. Absorbing material can beremoved in the vertical as well as in the horizontal direction.

Alternatively, absorber material, as for example chromium, may be addedin both vertical and horizontal directions. The deposition can also beperformed with an electron beam and/or with an ion beam and asappropriate together with a respective precursor gas or a combination ofprecursor gases. Both methods can be applied to correct surface defects380 and may be employed for a compensational repair of buried defects480 as schematically illustrated in FIG. 1.

A further repairing method comprises at least partially removing of themultilayer structure 305, 405, 605, as for example by drilling a holeinto the multilayer structure 305, 405, 605 having a suitable dimension.If the portion of the multilayer structure 305, 405, 605 which comprisesthe buried defect 480 is removed, the defect 480 is also removed. Then adefect-free multilayer structure 305, 405, 605 can newly be deposited onthe removed multilayer portion.

The effect of the repairing method described in the preceding paragraphis investigated by simulation prior to performing the replacement of aportion of the multilayer structure 305, 405, 605. This means, theintended correction is initially applied to the determined 3D errorstructure and a simulation of the resulting aerial images of the focusstack is performed. Only when the result confirms that the resultingaerial images of the corrected position have a sufficiently wide processwindow, the repairing method is really executed. If this is not thecase, further simulations are performed with varied repairing methods orrepairing parameters, respectively.

The parameters of a repairing method can be optimized by an iterativemethod. For example, if a buried defect 480 in a multilayer structure305, 405, 605 is to be repaired by drilling of a hole into themultilayer 305, 405, 605, simulations are performed for holes havingvarious depths, different diameters and/or having various distances fromthe buried defect.

For performing the above mentioned repairing methods an EUV maskinspection microscope is provided having an integrated repairing system.It is also possible to apply the defect analyzing and the defectrepairing in separate systems.

A further repairing method comprises a local compaction and/or a localexpansion of the substrate 310, 410 and/or of the multilayer structure305, 405, 605 by the impact of electromagnetic radiation. The substrate310, 410 and/or the multilayer structure 305, 405, 605 can for examplebe compacted or expanded by the usage of femtosencond laser pulses.

A compaction as well as an expansion of the various layers of an EUVoptical element can also be achieved by implanting ions at a suitableposition of an EUV optical element. In the example described in thefollowing, the method is discussed in the context of a volume expansion.However, it is appreciated that the discussion of a volume expansiondoes not restrict the described method to volume expansion. Rather, itis also applicable to perform a respective volume compaction.

FIG. 7 presents a curve which shows an induced change of the height of aphotolithographic mask as a function of the implanted ion dose. In theexample of FIG. 7, the mask was irradiated with a helium ion beam havinga beam energy of 30 keV. As already mentioned, the helium beam orgenerally an ion beam has basically two effects: (a) a sputteringeffect, i.e. (helium) ions collide with atoms of the sample and releaseatoms from the sample, (b) implantation, (helium) ions stay in thesample material.

In the example of FIG. 7, the implantation of helium atoms doesbasically not lead to a volume change below a built in ion dose ofapproximately 2·10¹⁶ cm². Above this value the implantation of heliumions results in a volume expansion of the mask material which goessteeply up with an increase of the implanted helium dose.

FIG. 7 illustrates the interrelationship between a volume expansion of aphotolithographic mask and the implanted ion dose for the example ofhelium ions. It is appreciated that similar curves also exist of otherinert gas atoms, in particular for noble gas atoms.

The focussed ion beam of an ion beam (FIB) apparatus can be used inorder to implant an ion dose in an EUV mask with a defined depthdistribution as well as with a defined lateral distribution of theimplanted ions. Alternatively, the FIB source can be integrated in themask inspection tool which is applied for the defect analysis asdiscussed above. Furthermore, an AFM can for example be utilized todetermine the volume change induced by the implanted ions. It is alsopossible to use one of the scanning tools or a combination of themmentioned in the context of the determination of the surfaceconfiguration in order to detect the local height change caused by theimplantation of ions into an EUV mask.

FIG. 8 shows a simulation of the depth distribution of a helium ion beaminto an EUV mask as a function of the ion energy. The helium beam hitsthe EUV mask at a position without an absorbing pattern element. Theordinate indicates the cumulated normalized sum of implanted ions. Ascan be seen from FIG. 2, the EUV mask has a thin capping layer 230 whichis denoted with the reference numeral 830 in FIG. 8. The multilayerstructure 805 has a width of approximately 280 nm. It is deposited on asubstrate 810 having a width in the millimetre range of which only theupper relevant portion is indicated in FIG. 8.

FIG. 9 presents a simulation of the depth distribution of a helium ionbeam into an EUV mask as a function of the ion energy. The helium beamhits the EUV mask at a position of an absorbing pattern element. Similarto FIG. 8, the ordinate indicates again the cumulated normalized sum ofimplanted ions. As indicated in FIG. 2, the absorbing pattern 940 has athin AR layer 945 of a few nanometers arranged on the absorbing patternelements 940. The absorbing pattern structure 940 has a width ofapproximately 70 nm. As already mentioned in the context of thediscussion of FIG. 2, depending of the absorber material, a thickness ofapproximately 50 nm is sufficient to basically absorb all incident EUVphotons. The multilayer structure 905 and the substrate 910 of FIG. 9are identical the multilayer 805 and the substrate 810 of FIG. 8.

As can be seen from FIGS. 8 and 9, the depth at which the ions areimplanted in the EUV mask depends on the energy with which the ions hiton the surface of the EUV mask. This means that the depth distributionof the implanted ions can be set by selected by the ion beam energy.Further, FIGS. 8 and 9 also reveal that the width of the depthdistribution also varies with the energy of the helium beam. Even forion beam energies of 90 keV the ions are implanted in a way so that morethan 50% of the implanted ions are integrated in a depth range of 200nm. For ion beam energies of 50 keV of less the majority of the build inions is implanted in a depth range of 100 nm or less.

A comparison of FIG. 8 and FIG. 9 shows that an absorbing patternelement does not significantly change the depth distribution of theimplanted helium ions.

The simulations of FIGS. 8 and 9 have been performed with the softwareprogram SRIM, which is described in the article “The Stopping and Rangeof Ions in Solids” by J. F. Ziegler, J. P. Biersack, and U. Littmark,Pergamon Press, New York, 1985.

The implanted dose distribution within an EUV mask can for example bedetermined by preparing cross-section samples of an EUV mask andinvestigating the samples with transmission electron microscopy (TEM).The relationship between an exposition of the EUV mask for apredetermined time with predetermined ion beam parameters and theresulting implanted dose distribution within the EUV mask can also beobtained for example from TEM measurements of a series of prepared testsamples. This means that the spatial distribution of the implanted ionscan be controlled by the selection of the ion beam parameters.

FIG. 10 schematically shows in the left partial image a pit defect 1020in a substrate 1020 of an EUV mask 1000. Similar to the FIGS. 3, 4 and 6the pit defect propagates as defect 1030 through the multilayerstructure 1005. The right partial image schematically shows themultilayer structure 1005 after the implantation of ions as for examplehelium ions. The helium ions are preferably integrated in Si layers 1040which causes a local increase of the volume of the Si layers 1040. Ithas been observed that the induced volume change varies as function ofthe ion species and ion absorbing material. Built-in helium ions inMo—Si layers predominantly result in a volume expansion of the Si layers1040. As a result, the local volume expansion of the Si layers 1040corrects the effect of the pit defect 1020 at the surface 1070 of themultilayer structure 1005.

In the example of FIG. 10, the correction of the pit defect 1020 isperformed by a local volume increase of individual Mo layers 1040 of themultilayer structure 1005. This repairing method results in a localbreach of the Bragg reflection condition. This problem can be avoided orat least reduced if the ions are preferably implanted in the lowestMo—Si layers of the multilayer structure 1005 which are close to thesubstrate 1010. As already explained above, the multilayer structure1005 shows an asymptotic reflective behaviour. The upper Mo—Si layersclose to the surface 1070 of the EUV mask moo contribute the majorportion to the reflected EUV radiation, whereas the contribution of thelowest Mo—Si layers is insignificant. Therefore, by correcting the pitdefect 1020 of the EUV mask moo by implanting ions in the lowest Mo—Silayers of the multilayer structure 1005 the local breach of the Braggreflection condition due to the repairing process has a minor effect onthe reflectivity of the multilayer structure 1005.

FIG. 11 schematically illustrates a further approach with allows therepairing of a defect buried in a multilayer without basically distortthe reflectivity of the multilayer structure. In the EUV mask 1100 aso-called capture layer 1115 is inserted between the substrate 1210 andthe multilayer structure 1105. The capture layer 1115 has a width ofapproximately several hundred nanometers. Suitable materials for acapture layer are for example silicon (Si) and/or molybdenum disilicide(MoSi₂).

It is the purpose of the capture layer 1115 to efficiently capture ionsand to provide a large local volume change. The diagram of the rightpartial image of FIG. 11 schematically presents the depth distributionof the implanted ions. In the thin capture layer 1115 the majority ofthe ions impinging on the multilayer 1105 is captured and integrated.FIG. 7 depicts that there is a threshold for the ion dose below which amaterial does not show a volume expansion. Therefore, the depthdistribution of the implanted ions (right partial image) indicates thatthe capture layer 1115 can provide a large local volume expansion whichcan be adjusted by controlling the beam parameters of the incident ionbeam. On the other hand, the doses implanted in the substrate 1110 andin the multilayer structure 1105 are not high enough in order to inducea volume change in these layers.

As already mentioned, the bombardment of a surface with an ion beamleads to a local removal of atoms from the material surface. This effectmay be detrimental to a multilayer as the upper Mo—Si layers provide animportant contribution to the overall reflectivity of the multilayerstructure.

FIG. 12 shows an EUV optical element 1200 which can comprise an EUV maskand/or an EUV mirror. The EUV optical element 1200 has as substrate 1210which has a pit defect 1220 and a multilayer structure 1205. Forsimplicity reasons the propagation of the buried defect through themultilayer structure 1205 is suppressed. Before starting the defectrepairing process by implanting of ions with an incident ion beam, alocal protection layer 1230 is deposited on the multilayer structure1205. As can be seen from a comparison of FIGS. 8 and 9, the use of athin protection layer does not significantly distort the spatialdistribution or depth distribution of the implanted ions.

The protection layer 1230 has a width of at least the lateral dimensionof the buried defect and a height of about 100 nm. Preferred materialsfor a protection layer 1230 are for example carbon (C) and/or tetraethylorthosilicate (TEOS). The protection layer 1230 is locally deposited onthe multilayer structure 1205 by using an electron bean and/or an ionbeam in combination with a precursor gas.

After the local deposition of the protection layer 1230, the burieddefect 1220 is corrected by implanting an appropriate ion dose in themultilayer structure 1205 as discussed above. The correction of theburied defect 1320 is schematically depicted in the middle part of FIG.12. The incident ions sputter a portion 1240 of the upper part of theprotection layer 1230. Therefore, the protection layer 1230 efficientlyprotects the surface of the multilayer structure 1205 from the sputteraction of the incident ions.

At the end of the ion implantation process, the protection layer 1230 isagain removed. This can for example be done by an etching process usingan electron beam and/or an ion beam together with one etching gas or acombination of etching gases.

Beside the sputter action at the surface, the interaction of the ionswith the material along their path through the material may inducedamages in the material along the ion path. Therefore, it can bebeneficial to guide the incident ions through portions of a multilayerstructure of an EUV optical element whose integrity is of lessimportance for the reflectivity of the multilayer film.

FIG. 13 schematically illustrates an example how such an improvement canbe realized for EUV mask 1300. Similar to the left partial image of FIG.12, the left partial image of FIG. 13 presents again a substrate 1310having a pit defect 1320. The propagation of the buried defect throughthe multilayer structure 1305 is again ignored. An absorbing pattern1340 is deposited on the multilayer film 1305. The buried defect 1320 isnot localised below an absorbing pattern element 1340, but below thesurface 1370 of the multilayer structure 1305. If an ion beamperpendicularly incidents on the buried defect 1320, it has to passthrough the surface 1370 of the multilayer 1305 and may damage thesurface 1370 by sputtering atoms from the Mo—Si layers closest to thesurface 1370. Furthermore, the ion beam might damage the multilayerstructure 1305 along its path to the buried defect 1320 as explained inthe preceding paragraph.

FIG. 13 illustrates a configuration with which a possible damage ofreflectivity critical portions of the multilayer can at least partiallybe avoided. The left partial image of FIG. 13 schematically shows theconfiguration at the beginning of the ion beam irradiation. The ion beam1380 obliquely impacts on the pattern element 1340 and obliquely crossesthe multilayer structure 1305 on its path to the buried defect 1320. Theright partial image of FIG. 13 schematically shows the situation at theend of the repairing process. The local volume expansion caused by theimplanted ions corrected the buried defect 1320. The surface 1360 of thepattern element 1340 is damaged by the sputter action of the ion beam1380 during the repairing process. If necessary, the surface damage 1360of the pattern element 1340 can be corrected by locally depositingabsorber material on the damaged surface 1360 by using a methoddescribed above.

The white area 1330 below the absorbing pattern element 1340 indicatesthe area with might be damaged by the interaction of the ions of the ionbeam 1380 with the Mo and Si atoms of the multilayer structure 1305. Ithas a drop shaped form. The major portion of the potentially damagedarea is located below the pattern element 1340. In particular, ions donot cross the Mo—Si layers closest to the surface 1370 of the multilayerstructure 1305. Thus, by the selected path of the ion beam 1380, theimpact of the ion beam 1380 on the reflectivity of the multilayerstructure 1305 is minimized.

An EUV mirror which does not have an absorber pattern can also becorrected with the discussed procedure if a protection layer 1230 ofFIG. 12 in arranged on the EUV mirror instead of the pattern element1340.

Finally, the discussed repairing methods can also be applied on defectsof a mask blank prior to the deposition of a multilayer structure. Bycorrecting defects existent on the substrate already on the mask blankpossible damages of the multilayer structure can be avoided.

The defined method for defect analysis reliably discriminates betweensurface defects and buried defects of EUV optical elements. Instead ofincompletely compensating the impact of buried defects of EUV masks bymodifying of the absorbing pattern, the discussed repairing methodcorrects buried defects by a modification of the substrate and/or of themultilayer structure of EUV optical elements.

The invention claimed is:
 1. A method for analyzing a defect of aphotolithographic mask for an extreme ultraviolet (EUV) wavelength range(EUV mask), the method comprising: a. generating at least one focusstack relating to the defect using an EUV mask inspection tool, whereinthe at least one focus stack comprises an image recorded in a focusplane, at least one image recorded above the focal plane, and at leastone image recorded below the focal plane; b. determining a surfaceconfiguration of the EUV mask at a position of the defect; c. providingmodel structures having the determined surface configuration which havedifferent phase errors and generating the respective focus stacks; andd. determining a three dimensional error structure of the EUV maskdefect by comparing the at least one generated focus stack of the defectand the generated focus stacks of the model structures.
 2. The method ofclaim 1, further comprising applying different repairing methods to thethree dimensional error structure and simulating respective focus stacksin order to determine an optimal repairing method.
 3. The method ofclaim 2, further comprising applying the optimal repairing method to thedefective position.
 4. The method of claim 2, wherein the repairingmethod comprises at least partially removing the multilayer structure,in particular drilling at least one hole into the multilayer structure.5. The method of claim 2, wherein the repairing method comprises locallycompacting and/or expanding the multilayer structure and/or a substrateof the EUV mask by locally focusing femtosecond laser pulses into theEUV mask.
 6. The method of claim 1, wherein the model structurescomprise an absorbing pattern structure on a surface of the EUV mask. 7.The method of claim 6, further comprising providing the absorbingpattern structure from EUV mask design data and/or from a recording ofat least one image.
 8. The method of claim 1, further comprising using arepairing method correcting the three dimensional error structure, sothat a resulting multilayer structure is at least approximatelycorrected to an ideal multilayer structure.
 9. The method of claim 8,further comprising applying the repairing method directly onto thedefective position of the EUV mask.
 10. An inspection microscope forphotolithographic masks in the extreme ultraviolet wavelength rangeadapted to: a. generate at least one focus stack relating to the defectusing an EUV mask inspection tool, wherein the at least one focus stackcomprises an image recorded in a focus plane and at least one imagerecorded above the focal plane and at least one image recorded below thefocal plane; b. determine a surface configuration of the EUV mask at aposition of the defect; c. provide model structures having thedetermined surface configuration which have different phase errors andgenerating the respective focus stacks; and d. determining a threedimensional error structure of the EUV mask defect by comparing the atleast one generated focus stack of the defect and the generated focusstacks of the model structures.